This disclosure relates generally to improved carrier compositions for use in electrophotographic imaging processes. In particular, this disclosure provides carrier compositions having improved impaction resilience, developer compositions including such improved carrier compositions, and image forming methods using such developer compositions.
The electrophotographic processes, and particularly the xerographic process, are well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development of the image with a developer, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive developer particles are selected depending on the development systems used. Moreover, appropriate triboelectric charging values associated with the aforementioned developer compositions are important, as these values enable continued constant developed images of high quality and excellent resolution.
Carrier particles in part consist of a roughly spherical core, often referred to as the “carrier core,” which may be made from a variety of materials. The core is typically coated with a resin. This resin may be made from a polymer or copolymer. The resin may have conductive material or charge enhancing additives incorporated into it to provide the carrier particles with more desirable and consistent triboelectric properties. The resin may be in the form of a powder, which may be used to coat the carrier particle. Often the powder or resin is referred to as the “carrier coating” or “coating.”
Various coated carrier particles for use in electrophotographic developers are known in the art. Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000, the disclosure of which is incorporated by reference herein in its entirety. These carrier particles may consist of various cores, including steel, with a coating thereover of fluoro-polymers and ter-polymers of styrene, methacrylate, and silane compounds.
U.S. Pat. No. 4,233,387, the disclosure of which is incorporated by reference herein in its entirety, illustrates coated carrier components for electrophotographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, U.S. Pat. No. 4,233,387 discloses coated carrier particles obtained by mixing carrier core particles of an average diameter of from about 30 microns to about 1,000 microns, with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320° F. to about 450° F. for a period of 20 minutes to about 60 minutes, enabling the thermoplastic resin particles to melt and fuse on the carrier core. While these developer and carrier particles are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics. Further, only specific triboelectric charging values can be generated, when certain conductivity values or characteristics are contemplated.
U.S. Pat. No. 4,937,166, the disclosure of which is incorporated by reference herein in its entirety, describes a carrier composition comprised of a core with a coating thereover comprised of a mixture of first and second polymers that are not in close proximity thereto in the triboelectric series. The core is described to be iron, ferrites, steel or nickel. The first and second polymers are selected from the group consisting of polystyrene and tetrafluoroethylene; polyethylene and tetrafluoroethylene; polyethylene and polyvinyl chloride; polyvinyl acetate and tetrafluoroethylene; polyvinyl acetate and polyvinyl chloride; polyvinyl acetate and polystyrene; and polyvinyl acetate and polymethyl methacrylate. The particles are described to have a triboelectric charging value of from about −5 to about −90 microcoulombs per gram.
U.S. Pat. No. 4,935,326, the disclosure of which is incorporated by reference herein in its entirety, discloses a carrier and developer composition, and a process for the preparation of carrier particles with substantially stable conductivity parameters, which comprises (1) providing carrier cores and a polymer mixture; (2) dry mixing the cores and the polymer mixture; (3) heating the carrier core particles and polymer mixture, whereby the polymer mixture melts and fuses to the carrier core particles; and (4) thereafter cooling the resulting coated carrier particles. These particulate carriers for electrophotographic toners are described to be comprised of core particles with a coating thereover comprised of a fused film of a mixture of first and second polymers which are not in close proximity in the triboelectric series, the mixture being selected from the group consisting of polyvinylidenefluoride and polyethylene; polymethyl methacrylate and copolyethylene vinyl acetate; copolyvinylidenefluoride tetrafluoroethylene and polyethylenes; copolyvinylidenefluoride tetrafluoroethylene and copolyethylene vinyl acetate; and polymethyl methacrylate and polyvinylidenefluoride.
U.S. Pat. No. 5,567,562, the disclosure of which is incorporated by reference herein in its entirety, describes a process for the preparation of conductive carrier particles which comprises mixing a carrier core with a first polymer pair and a second polymer pair, heating the mixture, and cooling the mixture, wherein the first and second polymer pair each contain an insulating polymer and a conductive polymer and wherein the carrier conductivity thereof is from about 10−6 to about 10−14 (ohm-cm)−1. The first polymer pair is preferably comprised of an insulating polymethyl methacrylate and a conductive polymethyl methacrylate, and the second polymer pair is preferably comprised of an insulating polyvinylidenefluoride and a conductive polyvinylidenefluoride.
U.S. Pat. No. 6,042,981, the disclosure of which is incorporated by reference herein in its entirety, discloses carriers including a polymer coating wherein the polymer coating may contain a conductive component, such as carbon black, and which conductive component, is preferably dispersed in the polymer coating. The conductive component is incorporated into the polymer coating of the carrier core by combining the carrier core, polymer coating, and the conductive component in a mixing process such as cascade roll mixing, tumbling, milling, shaking, electrostatic powder cloud spraying, fluidized bed, electrostatic disc processing or by an electrostatic curtain. After the mixing process, heating is initiated to coat the carrier core with the polymer coating and conductive component.
Efforts to advance carrier particle science have largely focused on the attainment of coatings for carrier particles to improve development quality and provide particles that can be recycled and that do not adversely affect the imaging member in any substantial manner. Many of the present commercial coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes causing failure upon impact or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate.
Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. High relative humidity hinders image density in the xerographic process, may cause background deposits, leads to developer instability, and may result in an overall degeneration of print quality. Triboelectric charges are usually lower under high temperature, high humidity conditions than under low temperature, low humidity conditions. It is desirable to have the measured triboelectric charges (tc) for a particular carrier used under high temperature, high humidity conditions and under low temperature, low humidity conditions, when entered into a ratio of (high temperature and high humidity)tc/(low temperature and low humidity)tc, to be close to 1.0 in order to obtain good development in high humidity.
In addition, mechanical stresses on developer compositions, including the carrier particles therein, remain problematic. Successive linear piles of developer material move along the exterior of the developer sleeve. As a result, the skiving blade periodically impacts the entire length of a linear pile of developer material, and there is a periodic and substantial increase in mechanical stress on the developer material, due to the rapid succession of developer pile masses encountered by the edge of the skiving blade. During each stress peak, the skiving blade, magnetic developer roll, developer material, along with the motor drive and any respective mechanisms including motor drive bearings, will experience a significant increase in mechanical force. Such skiving action can also include vibration between both the skiving blade and the shell, with resulting impact to the carrier particles, which shortens developer material life. For example, with respect to developer material containing carrier particles each of which are formed of a carrier core and a coating, the entire coating may separate from the carrier core as fragments in the form of chips or flakes, or the particles may fracture or otherwise fail upon impact, with subsequent sub-particles experiencing abrasive contact with machine parts and other carrier particles. These fragments, which generally cannot be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby yielding images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate.
As described above, there is a continuing need for carrier compositions having resilient coatings, and for developer compositions containing such carrier particles.